Method for bending tubes

ABSTRACT

A method of bending a tube within a tube, in which a smaller tube is sheathed with relatively incompressible solid substance. The sheathed smaller tube is then inserted into a larger tube, with the sheath substantially filling the space between the tubes. The tubes are bent together into a desired configuration and the solid substance of the sheath is removed, as by melting. In the preferred method, the sheath is extruded around the smaller tube, which is then inserted into the larger tube.

United States Patent 1 Hill et al.

[451 Sept. 4, 1973 METHOD FOR BENDING TUBES Inventors: Ernest W. Hill, Aurora; Melvin R.

Humphrey; Dale E. Maier, both of Evergreen; James H. Pope, Broomfield, all of Colo.

Assignee: Teledyne, Inc., Los Angeles, Calif.

Filed: May 1, 1972 Appl. No.: 248,823

US. Cl 72/46, 29/423, 72/369 Int. Cl B2ld 9/00 Field of Search 29/423, 455 R;

References Cited UNITED STATES PATENTS 9/1967 Berto et al. 29/423 3,488,827 1/1970 Aerts et al. 29/423 7 Claims, 2 Drawing Figures APPLY SHEATI-I TO SMALLER TUBE INSERT SHEATHED SMALLER TUBE INTO LARGER TUBE BEND TUBE WITHIN TUBE REMOVE SHEATI-I MATERIAL Patented Sept. 4, 1973 3,755,053

APPLY SHEATH TO SMALLER TUBE INSERT SHEATHED SMALLER TUBE INTO LARGER TUBE BEND TUBE WITHIN TUBE REMOVE SHEATH MATERIAL METHOD FOR BENDING TUBES BACKGROUND OF THE INVENTION There are many applications in which it is desirable to have a tube positioned within another tube, for example, to provide thermal insulation by means of the void between the tubes, or to provide for heat exchange between isolated fluids liquid or gaseous flowing in the tubes. Occasionally, a smaller tube carrying a hazardous fluid is encased in a larger tube to provide a margin of safety in the event of a rupture or leak in the smaller tube.

While it is a simple matter to insert a straight tube within another straight tube, it has been a difficult and expensive matter to produce a tube within a tube which is bent or curved. Such structures have been fabricated in the past by first bending the smaller tube to the desired shape and then encasing it be joining pre-cut sections of a larger tube around it. Another method employed heretofore is to insert an unbent smaller tube into a larger tube and then to fill the void between the tubes by pouring in a medium such as sand or a solidifiable liquid. After the bending is accomplished, the medium is removed.

The pouring of granular or liquid media into the space between concentric tubes requires that the smaller tube be positioned with care inside the larger one, usually by means of spacers placed at strategic points, and that the ends of the structure be capped or sealed to contain the medium during the process. Spacers may not be removable and those which remain in place often constitute undesirable obstructions to fluid flow through the completed structure and may also act as undesired paths for the flow of heat between the two tubes. The application of liquid seal end caps and spacers has generally required precision manufacture and hand labor, the cost of which may be tolerable in special purpose or custom work but which is unacceptable when the required structures must be fabricated in large quantities or at high production rates.

SUMMARY OF THE INVENTION In accordance with the principles of the invention, an unbent smaller tube, which is to be inserted into and bent within a larger tube to form a bent tube-within-atube structure, is sheathed with a solid substance to a thickness sufficient to substantially fill the gap between the tubes after assembly. The sheathed smaller tube is next inserted into the larger tube, with the sheath filling the space between the tubes and holding the smaller tube in the desired position inside the larger one.

After assembly of the unbent smaller tube within the larger one, the tubes are bent together into the desired configuration. Bending may be accomplished by any of the well-known tube bending techniques. After bending, the sheath material is removed from the space between the tubes, preferably by melting or solvent action or by a combination thereof.

The solid sheath may be applied to the smaller tube in the form of a hollow jacket which is slipped over its outer surface, or by extruding the sheath material around the smaller tube using convention extrusion machinery. Direct extrusion of the sheath onto the smaller tube is preferred for a high speed production.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block schematic diagram illustrating the method of the invention; and

FIG. 2 depicts a section of a smaller tube encased in a solid sheath and inserted into a larger tube, in accordance with the principles of the invention.

DESCRIPTION OF THE PREFERRED METHOD OF PRACTICING THE INVENTION The steps of the process which is the invention are illustrated in sequence in the block schematic diagram of FIG. 1. In the first step of the process, a sheath of a suitable material is applied to the smaller tube which is to be bent within a larger tube to form the final tube within a tube structure. The sheath will normally be applied to a straight or unbent section of the smaller tube. For applying the sheath to the large quantities of tube to be bent on a mass-production basis, the sheath is formed by extruding around the smaller tube as it passes through a conventional extrusion machine.

Next, sheathed sections of the smaller tube are inserted into sections of the larger tube to produce unbent sections of the tube within a tube structure. The thickness of the sheath having been selected to match the inside dimensions of the larger tube, the sheath material substantially fills the gap between the two tubes. The assembly of smaller tube, sheath and larger tube is then bent to the desired shape using conventional bending methods and apparatus. After the bending is accomplished, the sheath is removed from the space between the tubes by heating to melt the sheath material or by using an appropriate solvent or by a combination of melting and solvent action. Removal of the sheath material leaves the desired bent tube within a tube structure without any internal obstructions or thermal conducting paths resulting from internal spacers left over from the bending process. If desired, spacers may be provided at the ends of the structure to support the smaller tube within the larger one.

FIG. 2 depicts a section 20 of the sheathed tube within a tube prior to bending. The smaller, inner tube 21 is surrounded by a jacket or sheath 22 which has been formed by extrusion. The sheathed tube 21 has been inserted into a larger tube 23 which is of a preselected size and shape so that sheath 22 fills the gap between tubes 21 and 23 while holding them in the desired position with respect to each other prior to and during the bending procesS.

The material of the sheath 22 must have properties which permit it to be formed into a sheath of the required cross-section. In addition, the solid sheath must be capable of transferring the mechanical forces of bending between the tubes, and must be removable by either melting or solvent action. Additionally, it must not leave a residue which would interfere with the intended function of the bent tube within a tube. We have found that wax-base substances can be compounded which meet these requirements. One such material is commercially available from M. Argueso & Company of Mamaroneck, N.Y., under the trade name RIGIDAX.

RIGIDAX" is extrudable. It has a relatively low melting point so that the sheath may be removed from between the bent tube within a tube by moderate heating. Alternatively. RlGlDAX" may be removed with hot trichloroethylcne. The material may be reclaimed and used again in the method of the invention. At elevated temperatures, any residual RIGIDAX forms a harmless ash and about 0.003 parts per million of hydrochloric acid. Solid RlGlDAX is substantially incompressible and, since it maintains its density, effectively transmits bending forces between the tubes.

Other materials may, of course, be employed in practicing the invention, many variations and modifications of which are possible and may be made by those skilled in the art without departing from its scope and spirit.

What is claimed is: 1. A method of bending a tube within a tube comprising the steps of:

applying to the outer surface of an unbent section of a smaller tube a sheath of a solid liquefiable substance, the thickness of the sheath being sufficient to substantially fill the space between the tubes when the sheathed smaller tube is inserted into a larger tube of predetermined cross-section; inserting the sheathed section of the smaller tube into an unbent section of a larger tube of predetermined cross-section so that the sheath substantially fills the space between the tubes; bending the section of the tubes encompassing the sheath to a desired configuration; liquefying the sheath material; and removing the liquefied sheath material from the space between the tubes.

2. A method of bending a tube within a tube as in claim 1 wherein the sheath applied to .the smaller tube comprises a wax-base material.

3. A method of bending a tube within a tube as in claim 1 wherein the sheath applied to the smaller tube comprises an extrudable substance.

4. A method of bending a tube within a tube as in claim 3wherein the sheath is applied by extruding the substance around the smaller tube.

5. A method of bending a tube within a tube as in claim 3 wherein the substance is substantially incompressible in the solid phase.

6. A method of bending a tube within a tube comprising the steps of:

extruding around the outer surface of a smaller tube a sheath of a substantially incompressible solid substance;

inserting the sheathed smaller tube into a larger tube adapted to be substantially filled by the sheathed smaller tube;

bending the smaller and larger tubes together to a predetermined shape; and

removing the sheath material from the space between the smaller and larger tubes.

7. A method of bending a tube within a tube as in claim 6 wherein the sheath extruded around the smaller tube is a wax-base material. 

1. A method of bending a tube within a tube comprising the steps of: applying to the outer surface of an unbent section of a smaller tube a sheath of a solid liquefiable substance, the thickness of the sheath being sufficient to substantially fill the space between the tubes when the sheathed smaller tube is inserted into a larger tube of predetermined cross-section; inserting the sheathed section of the smaller tube into an unbent section of a larger tube of predetermined cross-section so that the sheath substantially fills the space between the tubes; bending the section of the tubes encompassing the sheath to a desired configuration; liquefying the sheath material; and removing the liquefied sheath material from the space between the tubes.
 2. A method of bending a tube within a tube as in claim 1 wherein the sheath applied to the smaller tube comprises a wax-base material.
 3. A method of bending a tube within a tube as in claim 1 wherein the sheath applied to the smaller tube comprises an extrudable substance.
 4. A method of bending a tube within a tube as in claim 3 wherein the sheath is applied by extruding the substance around the smaller tube.
 5. A method of bending a tube within a tube as in claim 3 wherein the substance is substantially incompressible in the solid phase.
 6. A method of bending a tube within a tube comprising the steps of: extruding around the outer surface of a smaller tube a sheath of a substantially incompressible solid substance; inserting the sheathed smaller tube into a larger tube adapted to be substantially filled by the sheathed smaller tube; bending the smaller and larger tubes together to a predetermined shape; and removing the sheath material from the space between the smaller and larger tubes.
 7. A method of bending a tube within a tube as in claim 6 wherein the sheath extruded around the smaller tube is a wax-base material. 